Hi, On Android, a JavaFX Application might transfer control to another app (and become invisible) and enter the foreground later. In that case, the GLSurface we are rendering on becomes invalid. In order to avoid problems and save battery, we want to pause the renderer thread, but this turns out to be more difficult than I expected.
When our app transfers control, we get a callback from Android. We intercept this in javafxports, and we set the Screen width/height to 0/0 as we don't want to render on the (invalid) surface while we lost control. When we regain control, we resize the Screen and the app renders again. The reason we set the width/height to 0/0 is because the PresentingPainter will call SceneState.isValid() and this returns false in case getWidth() or getHeight() are 0. However, SceneState extends PresentableState and it overrides the update method. It will call PresentatbleState.update() which will set the viewWidth to the width of the new Screen (hence, 0) , but after that it overwrites the viewWidth with camera.getViewWidth(). The latter still contains the old value. A quick inspection shows that camera.setViewWidth() is called when validate(...) is called on NGDefaultCamera, which is called by ES2Context.updateRenderTarget() which happens during rendering, hence *after* the PresentingPainter checks if the width is 0. So immediately after we set the width of the Screen to 0 (on the FX App Thread), a Pulse happens, and this one still things the screen is the original size. While the pulse is happening, the android system destroys our context, and the rendering fails. Moreover, the EGL system is in a unpredictable state (recreating the surface fails). A very dirty workaround for this is to wait for 1 pulse (with the new pulselistener API this should be possible) before we return from the callback method called by Android when the surface is about to be destroyed. That way, we will have 1 bogus rendering on an existing (but about-to-be-destroyed) surface. But it would be better if there is some way to tell the quantum renderer to immediately stop rendering. Existing pulses are no problem, as the renderLock guarantuees that we set the size to 0 only when no other thread (quantum renderer) has a lock on the renderLock. - Johan
